The mystery of how owls can rotate their necks almost 270 degrees without
suffering serious harm has been solved by scientists.

When humans attempt sudden and violent twists of their neck they risk damaging the lining of their blood vessels, which can result in a fatal blockage or stroke.

But owls are able to do so safely because of unique aspects of their bone structure and circulatory system which evolved to cope with their unusually large heads, scientists claim.

A study published in the Science journal found that blood vessels at the base of owls' heads are able to balloon outwards when blood flow increases.

This enables them to create "reservoirs" of blood which are needed to supply the animals' out-of-proportion heads and eyes while their heads are turned, researchers explained.

In contrast, humans' arteries tend to decrease in size and do not swell to accommodate greater blood flow, creating the risk of clotting after sudden neck movements such as whiplash or botched chiropractic therapy.

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Owls were also found to have especially large cavities in their neck vertebrae through which arteries pass, leaving extra space for movement when the neck is twisted, and to have small connections between the carotid and vertebral arteries which allowed blood to pass from one to the other in the case of one becoming blocked during extreme neck rotation.

The scientists reached their findings by injecting dye into the blood vessels of owls that had died of natural causes to mimic natural blood flow.

They found that when they turned the animals' heads, the blood vessels below the jaw bone expanded as more dye entered, creating pools of blood capable of maintaining the energy supply to the brain and eyes.

Dr Philippe Gailloud of Johns Hopkins University School of Medicine, senior investigator of the study, said: "Until now, brain imaging specialists like me who deal with human injuries caused by trauma to arteries in the head and neck have always been puzzled as to why rapid, twisting head movements did not leave thousands of owls lying dead on the forest floor from stroke."

The study results demonstrate what physical properties are needed to allow such extreme head movements, and explain why injuries sustained from chiropractic treatment can have such serious consequences for humans, he added.